Swingle Citrumelo Decline in Belize

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Swingle Citrumelo Decline in Belize Francisco Gutierrez Extension Coordinator Citrus Research and Education Institute Introduction Many citrus growers in certain areas of the country are observing a

1 Swingle Citrumelo Decline in Belize Francisco Gutierrez Extension Coordinator Citrus Research and Education Institute Introduction Many citrus growers in certain areas of the country are observing a decline of Valencia and grapefruit trees planted on Swingle Citrumelo rootstock. The condition starts as a slow decline of a few trees within the grove and with time is noticed in other trees. The first incidence was reported in a six-year old grove at a farm on the Western Highway early The development of the problem suggested that the cause was one of many diseases that can affect citrus. At the time it was suspected that Phytophthora was the cause of the problem because the initial symptoms occurred on trees planted in flood-prone areas. However Phytophthora foot rot was ruled out as a possible cause after inspection by a specialist who carried out visual inspections of uprooted trees. A nematode test was also done, ruling out the possibility of nematodes as the causal agents. Other factors were also considered as possible causes for the decline. CREI personnel eventually ruled out Tristeza and Blight as possible causes after conducting lab tests and water uptake tests respectively. Late in 2000 and early 2001, more growers in the area reported similar problems with the Swingle rootstock. Similar conditions have been reported in the clay soils of Toledo and some areas in the Stann Creek District. The problem became a matter of concern given that there are 2500 acres of citrus planted on Swingle rootstock in the industry, accounting for seven percent of the acreage. CREI staff investigated the affected sites, searched the literature and corresponded with Swingle rootstock experts in Florida. Initial findings indicated the probability of soil conditions as the cause of the problem. Below follows a description of the investigative process to determine the cause of the problem. General observations in the affected blocks The blocks affected with Swingle decline share a few characteristics. The ages are generally around six to seven years, generally planted on raised beds, and in most of the cases, the trees had been growing very well until the recent onset of the progressive decline. In general, the groves are not prone to stagnant water and water logged conditions. Adjacent blocks of Valencia and/or grapefruit on non Swingle rootstocks (such as Volkameriana lemon, sour orange, and Cleopatra Mandarin), all show vigorous growth and no decline symptoms. In the early years, the trees start off well, have a good growth rate and are precocious producers, starting production as early as two years. When these trees are six or seven years old, decline

2 starts. The trees start declining in patches and the condition spreads radially from these focal points. Visible symptoms of the condition include reduced tree vigor, nutrient deficiencies and a drastic drop in production. During summer, these trees tend to suffer much more from water stress. Over time, the tree then starts showing tip dieback, foliage drop and eventual death. Inspection of the root systems of trees at varying degrees of decline have shown that there is a gradual death of the feeder Poorly developed system of Swingle rootstock root system. The degree of dead roots is directly reflected in the declining canopy. The conditions, which are very similar in all the farms with the problem, occur on 5-8 year old trees on average. However the age of the affected groves ranges from three to ten years. Causes of citrus decline and the ruling out process This type of decline had not been seen in Belize before the introduction of this rootstock. Many conditions can cause decline of trees. The investigative process carried out by CREI staff involved ruling out the less likely possibilities for the decline. Graft transmissible diseases: Tree decline is characteristic of Tristeza and Blight. However, these possibilities were ruled out following tests for Blight and Tristeza. (Some Tristeza strains can affect citrus regardless of rootstock. However these strains are not likely to be present in Belize). If blight were responsible for the condition, then adjacent trees on Volkameriana Lemon, which was the case in a few groves, would be declining as well, since this particular rootstock is very susceptible to this disease. Swingle is considered highly tolerant to Blight. Poor nutrient status of the soil: The soil analyses results indicated that most nutrient levels were adequate with the exception of a few cases in which some particular nutrients were low. Most of the groves were fertilized on a yearly basis. However, when the decline started it was noticed that the trees responded very little to granular fertilizer applications. This led growers to discontinue fertilization of these groves. Low nutrient levels would cause deficiency symptoms and a drop in production but not severe decline except at extreme low levels. The leaf analyses results indicated low levels for most nutrients. These low nutrient levels of the leaves are indications of deficient root systems that are not able to take up the fertilizers applied to the ground. Waterlogging: Waterlogging cannot be the main reason for the condition. It was pointed out before that the declines generally start in flood prone, low lying areas. However, the decline spreads to higher areas where water is not stagnated even during heavy rains. Once the decline starts, it progresses through the rainy and dry seasons and is accentuated more by extreme dry conditions. This is logical since it shows that the root systems of the trees are greatly affected by

3 whatever adverse condition and this is manifested as a decline. Although most affected areas did not appear to be waterlogged, there were some indications of internal drainage problems. The most affected areas in the groves showed a proliferation of some species of sedges (Cyperaceae), which survive very well in constantly humid soil. According to literature, Swingle should tolerate wet conditions and flooding fairly well. Phytophthora Root Rot: We cannot rule out Phytophthora altogether since this can be a possible contributing factor to the problem. However, if Phytophthora rot was the main cause, then the decline would have started earlier in the lives of the trees, since young trees are more susceptible than adult trees. According to the literature, Swingle should be resistant to Phytophthora so this disease should not be a major problem. However, it has been noted that a few younger Swingle trees in Belize die and exhibit foot rot symptoms. Recent information from Florida indicates that Swingle is resistant to Phytophthora nicotianae but highly susceptible to the less common Phytophthora palmivora. It is unclear what species complex exists in Belize and this merits more investigation. Nematodes: Some nematode species cause a problem in citrus known as spreading decline, similar to what is observed in declining Swingle. However, there are a few differences between the two conditions that suggest that this is not the causal agent of the condition in Swingle Nematodes can be ruled out as a possible cause of the decline because of the following reasons: Swingle is susceptible to the burrowing nematode but resistant to the citrus nematode. The Burrowing Nematode is reported to be present only in Florida. Even if this nematode was present in Belize, it is unlikely that it would be present in all the groves with Swingle at the same time. Nematodes would affect other rootstocks as well and at varying ages, especially young plants. Nematodes move very slowly in soil, especially in clay soils, and the decline is relatively fast spreading. The management of a farm in the Cayo District conducted a nematode test and ruled a nematode infestation as a possible cause of Swingle decline. This analysis pointed out to adverse soil factors, as possible causes of the decline and a more detailed investigation on soils characteristics was required. Experiences with Swingle rootstock outside Belize Swingle has been used as a rootstock in Florida for much longer than in Belize and Florida growers have had similar problems associated with this rootstock. The soils in Florida are very sandy unlike the clayey soils in Belize. However, there are areas called sloughs and depressional areas that are characterized by a clay horizon that extends from twenty four inches below the soil surface. These are soils mostly in the Winder and Riviera series in Florida. These types of soils dominate the areas known as the Flatwoods. Swingle trees in these soils start to decline at about five to eight years of age, similar to the situation in Belize. The condition has been associated with clay layers within the soil profile, yet more work is being carried out to determine why trees die on these heavy soils. Swingle decline has been reported in Mexico and Costa Rica as well. Many other soil conditions affect the performance of Swingle rootstock, such as soils with a high calcium and copper content.

4 The development and importance of Swingle rootstock In 1907, Dr. T.W. Swingle, a plant breeder and researcher working for the United States Department of Agriculture (USDA), developed the variety now known as Swingle by crossing Trifoliate Orange (Poincirus trifoliata) with Duncan Grapefruit (Citrus paradisi). The USDA kept a specimen tree in their variety collection for over 40 years. In the early 1950 s the variety was included in a USDA rootstock trial and in 1960 it was included in three rootstock trials on Marsh grapefruit, Pineapple orange, and Valencia orange. The trees on Swingle produced high quality fruit and reasonable yields for their size. By coincidence, the three trials were placed in soil types that were suitable for the variety. None of these trials were placed in locations where problems were likely to happen, such as high ph, calcareous areas, heavy clays, wet areas, or soils that had a clay or organic pan that causes a stagnant water situation. In the early 1970 s the Florida citrus industry was suffering tremendous losses from Citrus Blight and Citrus Tristeza Virus (CTV). Enough information on Swingle had been collected to know it was tolerant to CTV and a check of the known trees on Swingle at the time turned up only two blight trees while trees on other rootstocks had suffered up to 90% loss. The decision in 1974 to release Swingle to the Florida industry was due to its low level of Blight susceptibility. It has been written that "due to Swingle's outstanding performance in Florida and South Africa it has risen to commercial status faster than any rootstock in history". For the last twelve years around 50% of the trees planted in Florida have been on Swingle. Most plantings in areas with clay soils turned out to be troublesome. The trees start off well (except those in calcareous soils), and grow extremely fast and are quite precocious producers. When the trees get larger, and the rooting area of the tree reaches the clay or pan layer, the roots are shaved off by perched water or prevented from penetrating deeper for water uptake The Swingle groves planted on the deep sandy soils are around twenty years of age and apparently performing very well. It must be noted that the production in Florida relies on irrigation for optimum production. Swingle was introduced into Belize as an alternative, Tristeza-resistance rootstock to Sour Orange and now accounts for about 7% of the planted acreage. The earliest plantings took place in early 90 s, probably as early as 1991, although a couple farms report plantings as early as The bulk of the older plantings are around seven to eight years of age. Early experience showed that the rootstock could not tolerate heavy soils and soils with waterlogged conditions and high calcium content. The recommendation for Swingle was to camber before planting and therefore, most plantings were done this way, apparently solving the problems of drainage. Results from the soil analyses done at various farms in the Cayo District To corroborate the findings and experiences in Florida, CREI personnel conducted soil profile checks in the affected areas. This consisted of digging pits to thirty inches deep and collecting samples of the different layers. Samples were collected at six inch intervals and sent for a texture analysis to determine the clay content of the different layers. Also soil and leaf samples were taken and sent for analysis to determine nutrient content. For comparison, a fifteen year old grove with non-declining Swingle was sampled. It should be noted that this particular grove is the oldest block on Swingle in the country. Most other declining groves sampled were no older than eight years and very few blocks older than this are non-declining. The results of the texture analyses consistently indicate that the soils in the declining groves have very high clay contents

5 way above the levels that the rootstock will tolerate. In most cases the top 12 to 18 inches present a lighter soil of lower clay content. In most of the cases, there is a sudden sharp change to a very hard clay layer ranging up to 70% clay content from 18 inches downwards (Table 1). Table 1: Texture analysis of six soils from the Cayo District with declining and non-declining Swingle groves Farm Texture Age Status 1 Sand Declining Silt Clay 38 * 30 * 66 * 68 * - 2 Sand Declining Silt Clay * 50 * 64 * 3 Sand Declining Silt Clay 41 * 43 * 49 * 51 * 51 * 4 Sand Declining Silt Clay 29 * 41 * 59 * 73 * 53 * Sand Declining Silt Clay 27 45* 49* 61* 6 Sand Healthy Silt Clay *ClaylayerstooheavyforSwingle. In some cases, there is a layer of higher clay content in the first six inches followed by a layer of lower clay content down to twelve inches. This happens because the second layer was the original layer and during cambering a second slightly heavier layer was added. The following layers in the profile are heavy soils and are called argilic layers because of the high clay content. According to the literature, Swingle will not perform very well in soils with clay contents above 25%. One of the sampled declining groves was on alluvial soils that had deep profiles and apparent excellent drainage. The soil test results indicated that the soil contained a high clay content layer at twenty four inches depth. This indicates that even good alluvial soils can have clay layers here in Belize and may be too heavy for Swingle. The only non-declining sampled grove had soils with a very different profile. The soils of this grove were alluvial deposits of a sandy loam top horizon followed by sandy lower horizons with little or no clay content. The soil analysis results showed in general medium to high levels of most nutrients with very few exceptions. Nitrogen, phosphorous and potassium were found to be at medium or low. Calcium and magnesium were in medium to high quantities, because of the basic nature of the

Cayo soils. The leaf analysis results indicated generally low nutrient levels. This is an indication that the root systems are affected to the point that nutrient uptake is minimal.

6 Cayo soils. The leaf analysis results indicated generally low nutrient levels. This is an indication that the root systems are affected to the point that nutrient uptake is minimal. Swingle does not tolerate high calcium and copper levels, which is a condition with some of the sampled soils. However, these conditions would cause poor tree performance but not outright decline. Situation with other trifoliate hybrids and trifoliate varieties Besides Swingle Citrumelo, other trifoliate hybrids are used as rootstocks in the Belize citrus industry. Carrizo Citrange and C35 Citrange (hybrids of Poincirus trifoliata and Citrus sinensis) are widespread in the industry, the former representing about 6% of the planted acreage. The latter is of minor importance in terms of acreage planted. Carrizo was once very popular but lost its appeal when vast acreages had to be replanted because of Citrus Exocortis Viroid infections due the use of non-certified budwood. The performance of these two rootstock varieties has been quite varied. There are cases of very good groves on both rootstocks as well as reports of declines. The declines are not as widespread as that of Swingle. However it is probably just a matter of time before these declines are manifested in a wider scale. Apparently these two rootstocks are less delicate and will tolerate much harsher soil conditions than Swingle will. The few declining blocks have been planted on very heavy clay soils. This indicates that these two trifoliate hybrid rootstocks are at risk of future decline. A third trifoliate variety, a dwarfing variety known as Flying Dragon (Poincirus trifoliata) is also being planted in small amounts. However, these plantings are still very young to evaluate the performance of the rootstock. Conclusions Declining C35 Citrange in clay soils Based on the present experiences and those elsewhere, especially in Florida, it is believed that the decline seen in these farms are due to the heavy clay layers in the soil profiles. The trees show good initial growth, which is due to the cambering that raises the soil level way above the clay layer. After five years the bulk of the root system starts hitting the compacted layer and decline symptoms start showing up. The trees that start declining first are the ones where the clay layer is closer to the surface as is the case in low lying areas. In the deeper soils the roots hit the clay layers at different times thus the decline is seen as being progressive within the grove. Most affected groves start declining at six years on cambered groves and as early as three years on non-cambered groves. This difference occurs because the cambering raises the soil level to several inches above the clay layers and adds a few years to the life of the trees. When declining trees are uprooted, these present poorly developed root systems and dead feeder roots. The tap root grows downward to about eighteen inches and then starts spreading laterally once it gets to the clay layer. The lateral roots then become the main roots sustaining the tree in the top twelve to eighteen inches of soil (Fig 1). It is now believed that the problem with Swingle is not due to excess water alone but a combination of extreme soil conditions. The soils are subject to high

7 Figure 1: Different scenarios of Swingle decline as related to soil type water tables that drown the root systems during the wet season. During the extreme dry spell, the water deficit also causes massive root death. The hard compacted layers are very efficient at keeping the bulk of the root system very close to the surface, rendering it very susceptible to quick drying up. In soils where the clay layers are a little deeper, the roots system penetrates much deeper and the decline is expressed at a much later time. As explained before, some soils with apparent good internal drainage, but with clay layers, also caused Swingle to decline. An explanation may be that these soils have a very low water holding capacity in the top layers of soil and trees will tend to suffer from water deficit during the dry spell. Even in these soils, the clay layers act as physical barriers that restrict the root systems to the top twelve to twenty four inches of soil. It is believed that the extreme dry conditions are responsible to a large extent for the gradual death of the root systems. It is also believed that the 2000 extreme dry season was a critical turning point for this condition in the groves (driest summer in 25 years in the Cayo District coinciding with the average age of decline for the majority of groves). Swingle cannot regenerate roots very well to replace dead ones under these conditions and the root system starts dying gradually. During the first years of growth, Swingle is a very vigorous rootstock. The young, vigorous root system tends to replace dead roots with relative ease after the many dry seasons. Little by little, these seasonal changes start taking a toll on the root system that gradually has to maintain a bigger canopy and eventually fruit production. The poor internal drainage during the rainy season and the water deficit and resulting water stress during the dry season cause the root systems to decline to an irreversible state and eventual death. What to do? Apparently very little can be done to stop the problem or reverse the conditions on declining trees. Some growers in Florida have reported that foliar fertilizer sprays, coupled with better irrigation, have been successful to a certain degree but it is clear that this helps the tree for a while but cannot totally reverse the condition. This approach does not solve the reduced water

8 uptake for the plant. In the case of Belize, it has been noticed that there is a progressive death of the roots, a condition that is impossible to reverse. Some growers in Florida have tried heavy applications of Ridomil (metalaxil) thinking that Phytophthora sp. Causes the problem but no positive reactions occur. Probably anything that will improve drainage will help the trees through varying degrees. Young declining groves less than three years old and young non-declining groves with potential of future decline may stand a chance if there is a change of rootstock using the inarching methodology. This could be a very expensive venture however, and the cost of inarching should be compared to the cost of replanting the whole grove. It is evident that Swingle is an excellent rootstock when planted under ideal conditions. Swingle performs very well if planted in well-drained light soils that are slightly acid. It is resistant to Tristeza, nematodes and blight and is a precocious producer of high quality fruit. Unless the appropriate soil requirements are met, the planting of new groves on Swingle for the Belize citrus industry is strongly discouraged. The experience with soils in Belize is that the majority of these present high clay contents in the profiles, even on those classified as alluvials. If growers still want to plant Swingle then they should consider this rootstock as a seven to ten year rotational crop at higher densities. Growers with declining groves should look into replanting these areas as soon as possible with a proven rootstock, such as Cleopatra Mandarin. Farmers who are interested in planting Swingle should thoroughly check the soil and drainage characteristics of each block and make sure it is suitable for Swingle, before making expensive mistakes.

UNITED STATES DEPARTMENT OF AGRICULTURE Agricultural Research Service Washington, D.C

UNITED STATES DEPARTMENT OF AGRICULTURE Agricultural Research Service Washington, D.C. 20250 NOTICE TO FRUIT GROWERS AND 1\u~SERYMENRELATIVE TO THE NAMING ANTI RELEASE OF THE US-897 CITRUS ROOTSTOCK The